This invention pertains to single-package zinc-rich coatings and more particularly to blends of zinc, partially hydrolyzed alkyl silicates and an aminosilane.
Zinc-rich coatings are effective in protecting steel against corrosion. The principle of this protective action is attributed to the fact that zinc, being higher than iron in the electromotive series of the elements reacts first in any environment conductive to the ionic dissolution (oxidation) of metals, thereby protecting the steel substrate.
As the name implies, zinc-rich coatings contain a high concentration of zinc in the dry film. This is required so as to provide the electrical continuity and, therefore, the conductivity necessary for the electrochemical process to take place.
In order to obtain these zinc-rich coatings on a ferrous substrate, a paint formulation containing very fine zinc dust produced by distilling the metal under controlled conditions of condensation is used. When the paint is applied, the metallic powder is held in place on the surface by a binder matrix. Zinc-rich coatings are classified, according to the nature of the binder, into organic or inorganic coatings.
Organic zinc-rich coatings utilize synthetic polymers as binders. Although such coatings afford effective corrosion protection, their heat and solvent resistance are limited.
Inorganic binders do not have these limitations. Such binders include water-soluble silicates, which are insolubilized by a curing composition after application, and alkyl silicates which do not require post cure. Although alkyl silicates contain organic chains, the resulting zinc-rich coatings are classified as inorganic because it is believed that, upon drying, a totally inorganic matrix of SiO.sub.2 is formed. This reaction takes place slowly and proceeds through continuous stages of hydrolysis. The alkyl silicates that may be used in zinc-rich coatings may vary in their level of hydrolysis. If an alkyl silicate having a very low level of hydrolysis is used, the curing reaction is so slow that the film remains uncured for prolonged periods of time. Using alkyl silicates hydrolyzed to higher levels reduces the time necessary to obtain dry films. Unfortunately as the drying time decreases due to the higher degree of hydrolysis, the stability of the product in turn decreases. This lower stability is manifested in various ways. One is an increasing tendency for the paint to gel in the container upon storage. Another is a diminished pot life when alkyl silicate is mixed with the zinc dust in which case gelling usually occurs in a few hours.
One way to avoid instability of the paint composition in the container and premature gellation with the zinc, is to package the zinc separately from the alkyl silicate and mix the two components just prior to application.
This is done commercially in the so-called 2-package zinc-rich coating compositions and the field has adapted its working methods to this characteristic of the product or paint used to provide the coatings. However, the problems inherent in a 2-component coating composition, viz., doubled production, warehousing, stocking and inventory, as well as metering and mixing on site coupled with limited pot life makes a single-component zinc-rich paint primer composition very desirable.
If zinc-rich coatings are made with alkyl silicates of low degrees of hydrolysis, stability of the alkyl silicate in its container as well as the pot life of primer composition after addition of the zinc dust to the alkyl silicate improves considerably. The price of this improved stability, however, is a much lengthened drying time. The problem facing the formulator is therefore how to obtain curing of a single-package alkyl silicate, zinc-rich paint primer composition in a reasonably short time, while maintaining good package stability in conjunction with non-reactivity of the alkyl silicate with the zinc dust.
Several proposed solutions for this problem have been put forth in the prior art. Thus, for example in U.S. Pat. No. 3,653,930 a single-package, zinc-rich coating was obtained by the addition of low molecular weight amines to ethyl silicate hydrolyzed to about 40% together with nitro compounds to prevent gassing. The same general approach was also described in Netherlands Pat. No. 6,900,729.
In U.S. Pat. No. 3,660,119 film formation of a 40% hydrolyzed alkyl silicate was obtained through the use of strong bases, such as, sodium or potassium methoxide or ethoxide.
U.S. Pat. No. 3,859,101 discloses use of zinc chromate instead of nitro compounds as anti-gassing additives in a mixture of alkyl silicate and zinc dust.
U.S. Pat. No. 3,917,648 utilizes a reaction product of alkyl silicates with polyols to form a product which is stable in the presence of zinc.
The prior art references recited above suffer the following disadvantages:
1. Low molecular weight amines are volatile and therefore alkyl binders containing them lose effectiveness upon storage.
2. Low molecular weight amines are water-soluble therefore introducing a factor of water sensitivity into a coating primarily intended for corrosion protection.
3. Low molecular weight amines have high chemical reactivity. Thus they react with acids such as those produced by absorbed carbon dioxide during storage. This may account for their loss of effectiveness with time.
4. Low molecular weight amines present in the coating formed on the ferrous substrate have adverse effects on the resistance of the zinc-rich film to environmental agents and interfere with the adhesion and chemical resistance of top coats applied to the primer coat.
5. Low molecular weight amines are toxic, representing a potential safety hazard to those coming in contact with the coating compositions.
6. Strong bases such as alkali metal alkoxides or their corresponding hydroxide by-products adversely affect a metal of amphoteric character such as zinc.
7. The alkali metal alkoxides or their corresponding hydroxide by-products remain in the zinc-rich film formed on the ferrous substrate, introducing an element of water and chemical sensitivity which may affect the performance of top coats applied to the primer coat.
8. Polyol silicate zinc-rich coatings produce films which are softer than desirable.
It is therefore an object of the present invention to provide a single-package zinc-rich coating composition containing an alkyl silicate which as a primer paint coating composition remains stable when packaged for prolonged periods of time. It is another object to provide coating compositions which upon application to a ferrous substrate rapidly form a dry, hard, corrosion-resistant protective primer film.